While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site catalysts represent the industry's future. These catalysts are often more active than Ziegler-Natta catalysts, and they often produce polymers with improved physical properties. However, in the production of linear low density polyethylene for film applications, Ziegler-Natta catalysts still predominate. One reason is that it has been difficult to process resins from single-site catalysts to produce polyethylene films with good properties, especially for thick films. Single-site polyethylene often processes poorly because of its narrow molecular weight distribution.
U.S. Pat. Nos. 6,232,260, 6,451,724, 6,559,251 and PCT Int. Appl. WO 01/53360 disclose the use of transition metal catalysts based upon indenoindolyl ligands. Indenoindolyl catalysts are remarkably versatile because substituent effects and bridging changes can often be exploited to provide polymers with tailored physical or mechanical properties. Non-bridged indenoindolyl complexes (as exemplified in the '260 patent) usually provide favorable activity although they sometimes fail to provide polymers having high enough molecular weights. Bridged indenoindolyl complexes (as taught, e.g., in U.S. Pat. No. 6,908,972) readily copolymerize α-olefins and provide polymers with varying levels of long-chain branching. Some of the examples of the '972 patent provide polymers with very low long-chain branching (see, e.g., Example 15, which reports no long-chain branching and Mw=90,700). For a discussion of long-chain branching in polyethylene, see Macromolecules 39 (2006) 1474 and references cited therein. Pending application Ser. No. 11/899,090, filed Sep. 4, 2007, and U.S. Pat. Nos. 7,655,740, 7,666,961, and 7,723,451 relate to slurry processes that provide polyethylene with varying levels of long-chain branching.
To produce medium or low density polyethylene, a catalyst system must have the ability to incorporate α-olefins. Some catalysts that incorporate α-olefins well also produce polyethylene with high levels of long-chain branching. Other catalysts can produce polyethylene with little or no long-chain branching. Long-chain branching has a pronounced effect on rheology. For some applications, it is desirable to have a moderate amount of long-chain branching. A desirable process would easily incorporate α-olefins in order to control density and other properties. Also important is the ability to produce polyethylene with high molecular weight.
U.S. Pat. No. 7,423,098 describes a bridged indenoindolyl complex for the slurry polymerization of ethylene in two reactor zones to produce polyethylene with a bimodal molecular weight distribution, a density of from 0.91 to 0.94 g/cm3, and a melt index of from 0.10 to 0.80 dg/min with good film properties. However, there is still a tradeoff between processability and film properties. Not disclosed is the use of a combination of a supported bridged zirconium complex and a supported non-bridged zirconium complex.
Multizone slurry polymerizations of ethylene with Ziegler-Natta catalysts are known. For example, U.S. Pat. No. 4,357,448 discloses a two-step process for polymerizing ethylene in the presence of a Ziegler-Natta catalyst in combination with a reaction product of a titanium or vanadium halogen-containing compound with a first reaction product of a Grignard reagent with a hydropolysiloxane. A small amount of a second α-olefin is optionally used and the lowest reported density is 0.9515 g/cm3. The reference does not teach how to make medium density or linear low density polyethylenes with good film properties.
U.S. Pat. No. 6,486,270 discloses a process to polymerize ethylene with a C3-C10 α-olefin in the presence of high levels of hydrogen to make polyethylene with a density of from 0.92 to 0.94 g/cm3 with multiple reaction zones using a Ziegler-Natta catalyst. The polyethylene has improved film properties versus high-density polyethylene, but the film properties are insufficient for many applications. For example, when the polyethylene is blown with a low stalk into a film having a thickness of 13 microns, the dart drop impact strength is less than about 100 grams per mil.
There has been some use of single-site catalysts in two reaction zones. U.S. Pat. No. 6,566,450 discloses a process using bis-indenyl single-site catalysts to produce polyethylene with a bimodal molecular weight distribution having a density of from 0.95 to 0.96 g/cm3 useful as pipe resin. Not taught is how to make medium density or linear low density polyethylene with good film properties.
U.S. Pat. No. 6,552,150 discloses a process which polymerizes ethylene in two reaction zones to give bimodal polyethylene with a density of 0.929 to 0.934 g/cm3 and good film properties. The low molecular weight portion has an MI2 greater than 100, which can cause problems with film homogeneity. Thick films are not prepared. The exemplified melt indices and blow-molding process conditions are not suitable for thick films. A Ziegler-Natta catalyst is preferred. The preferred process is a slurry loop reactor followed by a gas-phase reactor. While this process gives polyethylene with good properties, it would be desirable to produce polyethylene in an all-slurry or all-gas-phase process. A combination slurry and gas-phase process adds complexity and cost. The reference teaches that “while it may be possible to use a process comprising cascaded slurry reactors only, such a process is not recommended, due to problems which may occur when a component having a low density is dissolved in the reaction diluent.”
Heretofore, it has been difficult to achieve low densities with Ziegler-Natta catalysts in two reaction zones in a slurry process. Because of poor comonomer incorporation, waxes build up and can foul the reactor. Single-site catalysts are known to give improved comonomer incorporation, but they often cannot achieve the required molecular weight due to competing chain termination reactions and decompositions which produce hydrogen. The resultant polyethylene has inferior film properties. Often single-site catalysts that achieve the required high molecular weight also give high levels of long-chain branching. This can help processability by imparting good bubble stability in the film blowing process, but can have a deleterious effect on the impact strength. Another common tradeoff is between modulus and impact strength. Often, in order to obtain sufficient impact strength, the modulus must be low.
Films from polyolefin blends are known, but because property requirements vary with different applications and because film properties such as impact strength are based upon tradeoffs between processing, thickness, and modulus, further improvements are needed. U.S. Pat. No. 6,649,698 discloses blends of high molecular weight HDPE with LLDPE and their improved environmental stress crack resistance. The blends are used as geomembranes (polymer sheets used as environmental barriers) and pipes. Environmental stress crack data is given, but there are no other reported properties of the blends. Thick films are not disclosed.
Thin films have been studied for applications such as grocery sacks. U.S. Pat. No. 4,346,834 improves the thin film (preferably between 20 and 40 microns) properties of low density polyethylene (LDPE) by blending 5-20% by weight HDPE and LLDPE with the LDPE to provide a ternary blend. All of the blends contain LDPE and there is nothing disclosed about thick films.
U.S. Pat. No. 6,355,733 discloses a blend of LLDPE with medium density polyethylene having multimodal molecular weight distribution. The reported blends have a low modulus (examples range from 52,000 to 62,000 psi) and a low density (examples range from 0.927 to 0.931 g/cm3). The disclosure states that HDPE blends with LLDPE do not usually perform synergistically and solves this problem by using MDPE with multimodal molecular weight distribution.
Post reactor blending of polyethylene with LDPE improves melt strength and enables formation of thick films. Improved melt strength imparted by LDPE is not always necessary for thin films but is needed to blow thick films. Commercial thick films often contain 5-25% by weight LDPE. However, the LDPE is detrimental to modulus and impact strength. While HDPE can be added to increase the modulus, HDPE is also detrimental to impact strength.
In sum, there is a continuing need for a thick film that can be made without LDPE. There is a need for an all-slurry or all-gas-phase process that can provide medium or linear low density polyethylene that has the right balance of properties, i.e., the right amount of long-chain branching to enable good processability while making thick films with high modulus and good impact strength.